If C presented difficulties in terms of capability, efficiency, contribution,
debugging, etc., that would naturally drive us toward another language. But as
it stands, C is well understood, easy to resolve performance issues, there are
superb debugging and analysis tools available, and the runtime is exceedingly
mature.
And, as some have said, C code represents only a fraction of what Emacs is to
the user. Switching from Emacs Lisp to another high-level language might be a
more interesting discussion, which you could take up with the people working
on such projects, like Guile Emacs. If someone can prove to me that we gain a
lot and lose a little from such a switch, it's definitely worth considering!